Standardise whitespace to four spaces, not tabs

Hi,

Me again - just a quick whitespace standardisation, as I'd noticed whilst putting in my other PRs that the source files were a mix of tabs and spaces.

I've converted to spaces because a) they seemed more popular in the codebase and b) [Crockford said to](http://javascript.crockford.com/code.html#indentation) :).

Note: this will conflict with my other pull requests, but I'll happily rebase once these start getting merged. If you want to merge this one whilst you mull over the other ones, I'll rebase whilst you're considering them.

Thanks,
Rowan